The invention relates to electric field probe apparatus and methods and more particularly to nondestructive acoustic electric field probes capable of using an acoustic pulse of any shape to determine preselected characteristics of a material.
A recurrent problem in high-voltage engineering is measurement of electric fields inside insulating solids and liquids. The design, longevity and performance of high-voltage systems and their components depends upon accurate knowledge of such electric fields. The considerable effort spent on the determination of these electric fields may be divided into three categories.
A first category comprises modeling using a computer or devices such as electrolytic tanks, resistive paper and sense-electrode arrangements in air or oil. It will be appreciated by those skilled in the art that the knowledge gained is only as accurate as the models themselves, which are often not equipped to deal with unknowns such as space charge, transients and imperfections in the interiors of insulators.
A second category utilizes sense electrodes inside actual systems. This approach damages the device and perturbs the field patterns because it requires holes or other permanent changes in a system under test. Needless to say, this type of testing is of limited usefulness.
A third category encompasses nondestructive probes. For testing thin dielectrics, i.e., those less than about 100 .mu.m thick, several techniques are presently practiced including an electron-beam technique disclosed in Sessler et al., J. of Appl. Phys. 43, 408 (1972), acoustic step-discontinuity measurements as disclosed by Laurenceau et al., Phys. Rev. Lett. 38, 46 (1977), and thermal-pulse measurements as disclosed by both R. E. Collins in the J. Appl. Phys. 47, 4804 (1976) and DeReggi et al., Phys. Rev. Lett. 40, 413 (1978). While the techniques disclosed in these references are viable, they are useful only for measurements of electric fields in thin solid insulators. Non of these techniques lends itself to applications involving large insulating systems of arbitrary geometry or to alternating current measurements. The step-discontinuity acoustic probe of Laurenceau et al. might be applicable to large insulating systems but relies upon detecting the electric potential by utilizing a step-compression acoustic wave to generate an inhomogeneous deformation in an insulating solid. Because the measured quantity is the electric potential, small variations in the electric field of a large system under high electric stress may present almost insurmountable signal to noise problems. Furthermore, the accurate propagation of a step-compressional wave in insulators such as polymethylmethacrylate, polyethylene and other polymers is possible only for very short distances on the order of a few millimeters at best, because of the high attenuation of acoustic waves in these materials. Clearly, implementation of this type of device for large systems would be very difficult.
In practicing the invention, electric fields in insulators are measured utilizing a non-structured acoustic pulse to locally compress the dielectric of interest. Because pulse shape is unimportant in practicing the invention, attenuation effects are easily accomodated to effectively provide a probe range of tens of centimeters in polymers. Because the probe of the invention is sensitive to electric fields, small variations in electric field and space charge are detectable.